Geology and Neoarchean tectonic setting of the Central Hearne supracrustal belt, Western Churchill Province, Nunavut, Canada

The Central Hearne supracrustal belt, one of the largest Neoarchean “greenstone” terranes in the Canadian Shield, occurs in the Hearne domain of the Western Churchill Province, one of the largest, poorly known fragments of Archean crust on Earth. The belt contains two isotopically juvenile volcano–plutonic assemblages (I: ∼2710–2690 Ma, and II: ∼2685–2680 Ma), separated in time by localised, ∼2690 Ma, greenschist-facies deformation (D1), and overlain by ∼2680 Ma Archean siliciclastic and chemical sedimentary rocks. Extensive, penetrative, greenschist-facies regional deformation (D2) occurred at ∼2680 Ma, with amphibolite-facies metamorphism localised in the aureoles of isotopically juvenile synkinematic plutons. In many respects, the Central Hearne supracrustal belt is similar to other Neoarchean “greenstone” belts that have been interpreted in terms of arc-subduction systems, e.g. the Abitibi greenstone belt of the Superior Province. However, the principal tectonic characteristics of the Central Hearne supracrustal belt include: (i) location in an anomalously wide (>225 km) swath of penecontemporaneous juvenile crust that extends across much of the Hearne domain; (ii) close, primary intercalation of contemporaneous volcanic rocks of MORB-like and arc-like geochemical signatures, coupled with highly discontinuous volcanic map units; (iii) abundant intermediate to felsic volcanic rocks that do not represent a localised, laterally extensive volcanic arc edifice; and (iv) the development of isolated, independent, felsic volcanic centres throughout the magmatic history of the belt. These features are incompatible with oceanic arc or plateau models. We propose that the early history (assemblage I) of the Central Hearne supracrustal belt may be analogous with a modified extensional, suprasubduction “infant arc” model, such as that described for the earliest (Eocene) phase of construction of the Izu–Marianas–Bonin and Tonga arc-trench systems of the Southwest Pacific Ocean. The later history (assemblage II) may reflect attempted initiation of classical subduction and arc construction.